U.S. patent application number 10/608785 was filed with the patent office on 2004-12-30 for statistical adaptive-filter controller.
This patent application is currently assigned to Nokia Corporation. Invention is credited to Enzner, Gerald.
Application Number | 20040264686 10/608785 |
Document ID | / |
Family ID | 33540679 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040264686 |
Kind Code |
A1 |
Enzner, Gerald |
December 30, 2004 |
Statistical adaptive-filter controller
Abstract
This invention describes a statistical adaptive-filter
controller for digital acoustic echo control in hands-free
telephones for achieving more consistent echo cancellation results
(i.e. higher output signal quality) and simpler realizations of AEC
units. The improvement using the simple statistical adaptive-filter
controller is accomplished by optimizing a joint control of an echo
canceller and a postfilter.
Inventors: |
Enzner, Gerald;
(Neuhof/Zenn, DE) |
Correspondence
Address: |
WARE FRESSOLA VAN DER SLUYS &
ADOLPHSON, LLP
BRADFORD GREEN BUILDING 5
755 MAIN STREET, P O BOX 224
MONROE
CT
06468
US
|
Assignee: |
Nokia Corporation
|
Family ID: |
33540679 |
Appl. No.: |
10/608785 |
Filed: |
June 27, 2003 |
Current U.S.
Class: |
379/406.08 ;
379/406.01 |
Current CPC
Class: |
H04M 9/082 20130101 |
Class at
Publication: |
379/406.08 ;
379/406.01 |
International
Class: |
H04M 009/08 |
Claims
What is claimed is:
1. An echo cancellation system (11), comprising: a microphone (18),
responsive to an echo signal (22) from a loudspeaker (16) that
provides an acoustic output signal in response to a voice signal
(20), for providing an echo signal which is a component of a
microphone signal (28); and a statistical adaptive-filter
controller (40), responsive to the voice signal (20) and to an echo
reduced microphone signal (34), for providing a first control
signal (42) to an echo canceller module (21) and a second control
signal (44) to a postfilter (14); said control signals are provided
for optimizing cancellation of the echo signal.
2. The echo cancellation system (11) of claim 1, wherein the first
control signal (42) is a step-size signal which is used to
determine a gradient change of an echo transfer function signal
(15) provided to an echo canceller (10) of the echo canceller
module (21) according to a predetermined criteria.
3. The echo cancellation system (11) of claim 1, wherein the second
control signal (44) is a further transfer function signal of the
postfilter (14), said further transfer function signal weights an
echo reduced microphone signal (34).
4. The echo cancellation system (11) of claim 1, further comprising
the postfilter (14), responsive to an echo reduced microphone
signal (34) and to the second control signal (44), for providing an
output system signal (36).
5. The echo cancellation system (11) of claim 1, further comprising
the echo canceller module (21), responsive to the voice signal
(20), to the first control signal (42), and to an echo reduced
microphone signal (34), for providing an estimated echo signal (32)
to an adder (30).
6. The echo cancellation system (11) of claim 5, wherein the echo
canceller module (21) comprises an echo canceller (10) responsive
to the voice signal (20) and to an echo transfer function signal
(15), for providing an estimated echo signal (32) to the adder
(30).
7. The echo cancellation system (11) of claim 5, wherein the echo
canceller module (21) comprises a gradient adaptation means (12),
responsive to the voice signal (20), to the first control signal
(42), for providing for an echo transfer function signal (15) to
the echo canceller (10).
8. The echo cancellation system (11) of claim 5, further comprising
the postfilter (14), responsive to an echo reduced microphone
signal (34) and to the second control signal (44), for providing an
output system signal (36).
9. The echo cancellation system (11) of claim 1, further comprising
an adder (18), responsive to a microphone signal (28) and to an
estimate echo signal (32), for providing an echo reduced microphone
signal (34).
10. The echo cancellation system (11) of claim 1, wherein the
statistical adaptive-filter controller (40), the echo canceller
module (21) and the postfilter (14) operate in a time domain, and
said first and second control signals are provided in the time
domain as well.
11. The echo cancellation system (11) of claim 1, wherein the
statistical adaptive-filter controller (40), the echo canceller
module (21) and the postfilter (14) operate in a frequency domain,
and said first and second control signals are provided in the
frequency domain as well.
12. The echo cancellation system (11) of claim 1, wherein the
statistical adaptive-filter controller (40) and the echo canceller
module (21) operates in a time domain and the postfilter (14)
operates in a frequency domain, and the first control signal is
provided in the time domain and the second control signals is
provided in the frequency domain, respectively.
13. The echo cancellation system (11) of claim 11, wherein the
frequency domain is implemented as a Discrete Fourier Transform
(DFT) domain.
14. The echo cancellation system (11) of claim 13, wherein the
statistical adaptive-filter controller (40) is further comprising:
a first power spectral density means (40b), responsive to the voice
signal (20), providing for a first power spectral density signal
(46) of the voice signal (20); a second power spectral density
means (40c), responsive to an echo reduced microphone signal (34),
providing for a second power spectral density signal (48) of the
echo reduced microphone signal (34); and a statistical
adaptive-filter estimator (40a), responsive to the first and to the
second power spectral density signals (46, 48), providing for the
first and for the second control signals (42, 44).
15. The echo cancellation system (11) of claim 14, wherein the
first control signal (42) is a step-size signal which is used
according to a predetermined criteria to determine a gradient
change of an echo transfer function signal (15) provided to an echo
canceller (10) of the echo canceller module (21), said step-size
signal is determined according to: 5 ( k ) = G ' 2 XX ( k ) ee ( k
) ,wherein .vertline.G'.vertline..sup.2 is a predetermined constant
and .PHI..sub.xx(k) and .PHI..sub.ee(k) denote the first and second
power spectral densities signals (46, 48), respectively, and k is a
frame time index.
16. The echo cancellation system (11) of claim 15, wherein the
second control signal (44) is a further transfer function signal of
a postfilter (14), said further transfer function signal weights an
echo reduced microphone signal (34) and it is determined according
to: 6 H ( k ) = ee ( k ) - G ( k ) 2 XX ( k ) ee ( k ) ,wherein
.vertline.G(k).vertline..sup.2 is determined by solution of a
difference equation: .vertline.G(k+1).vertline..sup.2=.vert-
line.G(k).vertline..sup.2(1-2.mu.(k))+.mu.(k).vertline.G'.vertline..sup.2.
17. A method for acoustic echo control, comprising the steps of:
providing (100) an echo signal which is a component of a microphone
signal (28) of a microphone (18) which is responsive to an echo
signal (22) from a loudspeaker (16) that provides an acoustic
output signal in response to a voice signal (20); and providing
(108) a first control signal (42) to an echo canceller module (21)
and a second control signal (44) to a postfilter (14) by a
statistical adaptive-filter controller (40) which is responsive to
the voice signal (20) and to an echo reduced microphone signal (34)
for optimizing echo cancellation of the echo signal.
18. The method of claim 17, wherein the first control signal (42)
is a step-size signal which is used to determine a gradient change
of an echo transfer function signal (15) provided to an echo
canceller (10) of the echo canceller module (21) according to a
predetermined criteria.
19. The method of claim 17, wherein the second control signal (44)
is a further transfer function signal of a postfilter (14), said
further transfer function signal weights an echo reduced microphone
signal (34).
20. The method of claim 15, prior to the step of providing (108)
the first and second control signals (42, 44), further comprising
the step of: determining (104, 106) the first and the second
control signals by a statistical adaptive-filter controller
(40).
21. The method of claim 20, further comprising the steps of:
determining (112) an estimated echo signal (32) by the echo
canceller module (21) using the first control signal provided by
the statistical adaptive-filter controller (40); and determining
(114) an echo reduced microphone signal (34) by an adder (28) by
adding the estimate echo signal (32) to a microphone signal
(18).
22. The method of claim 21, further comprising the steps of:
determining (116) an output system signal (36) by the postfilter
(14) using the second control signal provided by the statistical
adaptive-filter controller (40).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application discloses subject matter which is also
disclosed and which may be claimed in co-pending, co-owned
application Ser. No. ______ (Att. Doc. No 944-003.178) filed on
even date herewith.
FIELD OF THE INVENTION
[0002] This invention generally relates to a digital Acoustic Echo
Control (AEC) in telephones and more specifically to introducing an
additional statistical adaptive-filter controller for achieving
more consistent echo cancellation results (i.e. higher output
signal quality) and a simpler realization of AEC units than
before.
BACKGROUND OF THE INVENTION
[0003] 1. Field and Background of the Invention
[0004] The invention is related to a digital Acoustic Echo Control
(AEC) unit of telephones. The purpose of the AEC is to prevent the
far-end speaker's speech circulating back as an echo after coming
out from the near-end phone user's loudspeaker and partly picked up
by the phone's microphone. A general concept is illustrated in FIG.
1 where "i" denotes the sampling time index. Advanced AEC units
contain an echo canceller module 21, generally consisting of an
echo canceller 10 with a gradient adaptation means 12, and a
postfilter 14 for residual echo suppression.
[0005] The need of an AEC unit in the hands-free telephones
basically arises from an acoustic echo path with an impulse
response h(i) from a local loudspeaker 16 to a local microphone 18.
The objective of the echo canceller 10 with an impulse response
w(i) is to find a replica of the echo path in order to compensate
for an echo signal d(i) 22 of a voice signal x(i) 20 received by a
loudspeaker 16 that provides an acoustic output signal in response
to the voice signal x(i) 20, thus generating in the microphone 18
the echo signal d(i) 22 which is one of the components of a
microphone signal y(i)=d(i)+s(i)+n(i) 28, where y(i) is a
microphone speech signal and n(i) is a background noise signal. As
the system identification process is always performed in the
presence of observation noise (local speech plus background noise)
s(i)+n(i), the objective of w(i)=h(i) cannot be reached exactly.
The echo canceller 10 generates an estimated echo signal d'(i) 32
which is negatively added to the microphone signal 18 by an adder
30 which generates an echo reduced microphone signal e(i) 34
containing the partially compensated echo signal. The echo reduced
microphone signal e(i) 34 is further provided to the gradient
adaptation means 12 and to the postfilter 14. The gradient
adaptation means 12 further provides a control signal 15 to the
echo canceller 10 by determining a gradient of the controlled
signal based on a predetermined criteria using the voice signal
x(i) 20 and the echo reduced microphone signal e(i) 34 as input
signals. The purpose of the postfilter 14 is further reducing of
residual echo components of the echo reduced microphone signal e(i)
34. The resulting output system signal s'(i)+n'(i) 36 after
residual echo suppression by the postfilter 14 is then transmitted
to the far speaker.
[0006] The basic principles of how to generate and control the echo
canceller 10 and the postfilter 14 are well known. However, there
are some problems involved in controlling them efficiently in a
most optimal way. The key variable in the whole control issue is
the residual echo, b(i)=d(i)-d'(i) which, unfortunately, cannot be
directly determined since it is inherently embedded in the echo
reduced microphone signal e(i)=b(i)+s(i)+n(i) 34.
[0007] The echo canceller module 21 of FIG. 1 often provides an
insufficient estimate d'(i) of the echo signal d(i) 22. The
postfilter 14 in the sending path of the telephone performs
residual echo suppression, but in many solutions this is achieved
at the cost of distortions (attenuations) of the useful signal
s(i)+n(i). In an alternative solution, the echo canceller module 21
can be used alone without a postfilter 14. This approach does not
introduce noticeable signal distortions, but normally requires very
sophisticated control mechanisms for the echo canceller. A more
simple and effective approach is needed.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is to an additional
statistical adaptive-filter controller for achieving more
consistent echo cancellation results and a simpler realization of
an acoustic echo control in telephones.
[0009] According to a first aspect of the present invention, an
echo cancellation system, comprises a microphone, responsive to an
echo signal from a loudspeaker that provides an acoustic output
signal in response to a voice signal, for providing an echo signal
which is a component of a microphone signal; and a statistical
adaptive-filter controller, responsive to the voice signal and to
an echo reduced microphone signal, for providing a first control
signal to an echo canceller module and a second control signal to a
postfilter; said control signals are provided for optimizing
cancellation of the echo signal.
[0010] In further accord with the first aspect of the invention,
the first control signal may be a step-size signal which is used to
determine a gradient change of an echo transfer function signal 15
provided to an echo canceller 10 of the echo canceller module 21
according to a predetermined criteria.
[0011] Still according to the first aspect of the invention, the
second control signal may be a further transfer function signal of
the postfilter, said further transfer function signal weights an
echo reduced microphone signal.
[0012] According still further to the first aspect of the
invention, the echo cancellation system further comprises the
postfilter, responsive to an echo reduced microphone signal and to
the second control signal, for providing an output system
signal.
[0013] Still further according to the first aspect of the
invention, the echo cancellation system further comprises the echo
canceller module, responsive to the voice signal, to the first
control signal, and to an echo reduced microphone signal, for
providing an estimated echo signal to an adder. Further, the echo
cancellation system may comprise an echo canceller, responsive to
the voice signal and to an echo transfer function signal, for
providing an estimated echo signal to the adder. Still further, the
echo cancellation system may comprise a gradient adaptation means,
responsive to the voice signal, to the first control signal, for
providing for an echo transfer function signal to the echo
canceller. Also further the echo cancellation system comprising the
echo canceller module, further comprises the postfilter, responsive
to an echo reduced microphone signal and to the second control
signal, for providing an output system signal.
[0014] Further still according to the first aspect of the
invention, the echo cancellation system further comprises an adder,
responsive to a microphone signal and to an estimate echo signal,
for providing an echo reduced microphone signal.
[0015] In further accordance with the first aspect of the
invention, the statistical adaptive-filter controller, the echo
canceller module and the postfilter may operate in a time or in a
frequency domain, and said first and second control signals are
provided in the time or in the frequency domain, respectively.
[0016] Yet further still according to the first aspect of the
invention, the statistical adaptive-filter controller and the echo
canceller module operates in a time domain and the postfilter
operates in a frequency domain, and the first control signal is
provided in the time domain and the second control signals is
provided in the frequency domain, respectively.
[0017] According further to the first aspect of the invention, the
statistical adaptive-filter controller, the echo canceller module
and the postfilter operate in a frequency domain, and said first
and second control signals are provided in the frequency domain as
well, wherein said frequency domain is implemented as a Discrete
Fourier Transform (DFT) domain. Further, statistical
adaptive-filter controller implemented in the DFT domain may
comprise a first power spectral density means responsive to the
voice signal, providing for a first power spectral density signal
of the voice signal; a second power spectral density means
responsive to an echo reduced microphone signal, providing for a
second power spectral density signal of the echo reduced microphone
signal; and a statistical adaptive-filter estimator, responsive to
the first and to the second power spectral density signals,
providing for the first and for the second control signals. Still
further, examples of calculating a step-size signal as the first
control signal and a further transfer function signal as the second
control signal are presented.
[0018] According to a second aspect of the invention, a method for
acoustic echo control, comprises the steps of: providing an echo
signal which is a component of a microphone signal of a microphone
which is responsive to an echo signal from a loudspeaker that
provides an acoustic output signal in response to a voice signal;
and providing a first control signal to an echo canceller module
and a second control signal to a postfilter by a statistical
adaptive-filter controller which is responsive to the voice signal
and to an echo reduced microphone signal for optimizing echo
cancellation of the echo signal.
[0019] According further to the second aspect of the invention, the
first control signal may be a step-size signal which is used to
determine a gradient change of an echo transfer function signal
provided to an echo canceller of the echo canceller module
according to a predetermined criteria.
[0020] Further according to the second aspect of the invention, the
second control signal is a further transfer function signal of a
postfilter, said further transfer function signal weights an echo
reduced microphone signal.
[0021] Still further according to the second aspect of the
invention, the method further comprises the steps of: coupling a
sidetone adaptive signal to an earpiece during the phone call; and
providing a sidetone sound audio signal to the user.
[0022] Further still in accordance with the second aspect of the
invention, prior to the step of providing the first and second
control signals the method further comprises the step of
determining the first and the second control signals by a
statistical adaptive-filter controller. Further, after the step of
determining the first and the second control signals, the method
further comprises the steps of further comprising the steps of:
determining an estimated echo signal by the echo canceller module
using the first control signal provided by the statistical
adaptive-filter controller; and determining an echo reduced
microphone signal by an adder by adding the estimate echo signal to
a microphone signal. Still further, after the step of determining
an echo reduced microphone signal by an adder, the method further
comprises the step of determining an output system signal by the
postfilter using the second control signal provided by the
statistical adaptive-filter controller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a fuller understanding of the nature and objects of the
present invention, reference is made to the following detailed
description taken in conjunction with the following drawings, in
which:
[0024] FIG. 1 is a block diagram representing a system for acoustic
echo cancellation.
[0025] FIG. 2 is a block diagram representing a system for acoustic
echo cancellation using a statistical adaptive-filter controller,
according to the present invention.
[0026] FIG. 3 is a block diagram representing a system for acoustic
echo cancellation implemented in the Discrete Fourier Transform
(DFT) domain using a statistical adaptive-filter controller,
according to the present invention.
[0027] FIGS. 4a, 4b, and 4c show construction of blocks 10, 12 and
14 of FIG. 3, respectively.
[0028] FIG. 5 is a flow chart illustrating a performance of a
statistical adaptive-filter controller of FIGS. 3 and 4 to optimize
echo cancellation, according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0029] This invention generally discloses a statistical
adaptive-filter controller for digital acoustic echo control in
hands-free telephones for achieving more consistent echo
cancellation results (thus a higher output signal quality) and
simpler realizations of AEC units than before.
[0030] The simple statistical adaptive-filter controller is
optimized for the joint control of an echo canceller module 21 and
a postfilter 14 of FIG. 1. The joint control of the echo canceller
module 21 and the postfilter 14 is not only simpler than the
individual optimization of the echo canceller module 21 and
postfilter 14, it also delivers more consistent results and a
higher output signal quality. The simple statistical
adaptive-filter controller is only partially useful for the echo
canceller module 21 alone.
[0031] FIG. 2 shows a block diagram representing an acoustic echo
cancellation system 11 using a statistical adaptive-filter
controller (SAFC) 40, according to the present invention. The SAFC
40 is basically the missing link between the echo canceller module
21 for acoustic echo cancellation and the postfilter 14 for
residual echo suppression. The blocks 21 and 14 are described above
in regard to FIG. 1. As shown in FIG. 2 the SAFC 40 provides a
first control signal 42 to a gradient adaptation means 12 of the
echo canceller module 21 and a second control signal 44 to the
postfilter 14 to perform fast and robust adaptation even in the
presence of a noise n(i). The SAFC 40, according to the present
invention, is obtained from a purely statistical optimization
process and is therefore extremely simple and robust. As shown in
FIG. 2, the SAFC 40 uses a voice signal x(i) 20 and an echo reduced
microphone signal e(i) 34 as input parameters.
[0032] The first control signal 42 can be a step-size signal which
is used to determine according to a predetermined criteria a
gradient change of a further control signal 15. According to the
present invention, the gradient adaptation means 12 provides the
further control signal 15 to an echo canceller 10 of the echo
canceller module 21 by determining a gradient of the further
controlled signal 15 based on a predetermined criteria using the
voice signal x(i) 20 and the echo reduced microphone signal e(i) 34
as input signals (as in FIG. 1), and additionally the first control
signal 42 from the SAFC 40.
[0033] The second control signal 44 can be a further transfer
function signal of a postfilter 14, said further transfer function
signal weights an echo reduced microphone signal 34 for generating
a high quality undistorted (strongly echo reduced) microphone
signal 36. The echo cancellation system 11 can operate in a time
domain or in a frequency domain. This implies that the statistical
adaptive-filter controller 40, the echo canceller module 21 and the
postfilter 14 can operate in the time or frequency domain, and the
first and second control signals can be also provided in the time
or frequency domain, respectively.
[0034] FIG. 3 shows an example illustrating a block diagram
representing a system for the acoustic echo cancellation
implemented in a Discrete Fourier Transform (DFT) domain, according
to the present invention. A stream of signal samples is processed
on a frame by frame basis in this approach. The signal frames are
obtained by the "windowing" operation and "k" is the frame time
index. The SAFC 40 provides the step-size signal .mu.(k) 42 to the
gradient adaptation means 12 of the echo canceller module 21. The
step-size signal .mu.(k) 42 is used to estimate a gradient
.DELTA.W(k) of the further control signal W(k+1)=W(k)+.DELTA.W(k)
15 according to the amount of observation noise in the microphone
as discussed below. The step-size signal .mu.(k) 42 is thus
responsible for the robustness and operation accuracy of the echo
canceller 10. The optimum step-size signal .mu.(k) 42 in a minimum
mean-square error (MMSE) sense can be found e.g. in G. Enzner, R.
Martin, and P. Vary, in Partitioned Residual Echo Power Estimation
for Frequency-Domain Acoustic Echo Cancellation and Postfiltering,
European Trans. on Telecommunications, vol. 13, no. 2, pp. 103-114,
March-April 2002, as the following ratio: 1 ( k ) = G ( k ) 2 XX (
k ) ee ( k ) , ( 1 )
[0035] wherein .PHI..sub.xx(k) and .PHI..sub.ee(k) are power
spectral density (PSD) signals of the signals x(i) 20 and e(i) 34,
respectively, and .vertline.G(k).vertline..sup.2 is a residual echo
power transfer function corresponding to the residual echo impulse
response g(i)=h(i)-w(i), where h(i) and w(i) are impulse responses
of an acoustic echo path and the echo canceller 10,
respectively.
[0036] The SAFC 40 further provides the second control signal 44, a
postfilter weights signal H(k) 44, to the postfilter 14 which is to
be applied to the echo reduced microphone signal e(i) 34. The
postfilter weights signal H(k) 44 is the further transfer function
signal (defined in comments to FIG. 2) of the postfilter 14 in the
frequency domain. The postfilter weights signal H(k) 44 is
responsible for the efficient suppression of the residual echo
components in the echo reduced microphone signal e(i) 34, thereby
not introducing audible distortions of the useful signal part
s(i)+n(i). The optimum postfilter weights signal H(k) 44 in the
MMSE sense is given by the Wiener filter in the DFT domain
described by 2 H ( k ) = ee ( k ) - G ( k ) 2 XX ( k ) ee ( k ) . (
2 )
[0037] The postfilter weights signal H(k) 44 depends on the same
parameters as the step-size signal .mu.(k) 42, including the
residual echo power transfer function
.vertline.G(k).vertline..sup.2 which is determined below, according
to the present invention. It is followed from Equations (1) and (2)
that .mu.(k)+H(k)=1 which is consistent with conclusions of E.
Hnsler and G. U. Schmidt, Hands-Free Telephones--Joint Control of
Echo Cancellation and Postfiltering, Signal Processing, vol. 80,
no. 11, pp. 2295-2305, 2000.
[0038] It has been observed in the theory and study of adaptive
filters (S. Haykin, Adaptive Filter Theory, Prentice Hall, 2002; A.
Mader, H. Puder, and G. U. Schmidt, Step-Size Controlfor Acoustic
Echo Cancellation Filters--An Overview, Signal Processing, vol. 80,
no. 9, pp. 1697-1719, September 2000; G. Enzner, R. Martin, and P.
Vary, Partitioned Residual Echo Power Estimation for
Frequency-Domain Acoustic Echo Cancellation and Postfiltering,
European Trans. on Telecommunications, vol. 13, no. 2, pp. 103-114,
March-April 2002) that it is extremely difficult to find a reliable
estimate of the residual echo power transfer function
.vertline.G(k).vertline..sup.2 required for the implementation of
Equations (1) and (2). According to the present invention a simple
statistical estimator for .vertline.G(k).vertline..sup.2 is used as
described in the following paragraphs.
[0039] The optimum step-size signal 42 in Equation (1) can be
estimated as 3 ( k ) = G ' 2 XX ( k ) ee ( k ) ( 3 )
[0040] wherein .vertline.G'.vertline..sup.2 is a pre-selected
constant. It can be shown theoretically that the specific choice of
the .vertline.G'.vertline..sup.2 results in an Echo Return Loss
(ERL) of -10 log 10(.vertline.G'.vertline..sup.2) dB between the
voice signal x(i) 20 and the echo reduced microphone signal e(i)
34. Therefore, the pre-selected value of
.vertline.G'.vertline..sup.2 can be understood as a target
convergence (target accuracy) of the echo controller module 21.
[0041] Given the step-size signal .mu.(k) 42 estimated using
Equation (3), it is then possible to perform a statistical
convergence analysis of the echo canceller 10 along the methodology
described by S. Haykin, in Adaptive Filter Theory, Prentice Hall,
2002. The result is a time-varying first order difference equation
for the residual echo power transfer function
.vertline.G(k).vertline..sup.2:
.vertline.G(k+1).vertline..sup.2=.vertline.G(k).vertline..sup.2(1-2.mu.(k)-
)+.mu.(k).vertline.G'.vertline..sup.2 (4).
[0042] Equation (4) only depends on the choice of the target
convergence .vertline.G'.vertline..sup.2 and the approximated
step-size signal .mu.(k) 42. Given some initial condition, Equation
(4) can be solved recursively at each frame index "k" for the
unknown value of the residual echo power transfer function
.vertline.G(k).vertline..sup.2. The solution can be used to
determine the postfilter weights signal H(k) 44 using Equation
(2).
[0043] The approximation of the step-size signal .mu.(k) 42 in
Equation (3) is extremely simple and therefore the echo canceller
10 is certainly working sub-optimum. Given the sub-optimum echo
canceller 10, the postfilter 14, according to Equations (4) and (2)
is however statistically nearly optimum in the MMSE sense.
Therefore, the postfilter 14 can correct weaknesses of the echo
canceller 10 to some extent.
[0044] Thus the SAFC 40 shown in FIG. 3 for implementation of an
algorithm described by Equations (2)-(4), comprises a first power
spectral density (PSD) means 40b which provides a first power
spectral density signal .PHI..sub.xx(k) 46 of the voice signal x(i)
20, a second power spectral density (PSD) means 40c which provides
a second power spectral density signal .PHI..sub.ee(k) 48 of the
echo reduced microphone signal e(i) 34, and a statistical
adaptive-filter estimator (SAFE) 40a which responds to the first
and to the second power spectral density signals .PHI..sub.xx(k) 46
and .PHI..sub.ee(k) 48, respectively, and determines and provides
the first and the second control signals 42 and 44 using Equations
(2)-(4). The SAFE 40a also pre-selects constant
.vertline.G'.vertline..sup.2. Windowing function for determining
signal frames is included in the PSD blocks 40b and 40c.
[0045] FIGS. 4a, 4b, and 4c show construction of blocks 10, 12 and
14 of FIG. 3, respectively. Construction of these blocks is
well-known to a person skilled in the art and is shown here for
illustration. Windowing of the signals x(i) 20 and e(i) 34 is
implemented using blocks 50 and 60, and 70 and 80, respectively.
DFT is performed by blocks 52, 62 and 82, Inverse Discrete Fourier
Transform (IDFT) is performed by blocks 56 and 82, and
multiplication operation is performed by blocks 54, 64, 66 and 84,
respectively. The gradient of the further controlled signal 15 in
FIG. 4b is determined using a normalized least-mean-square (NLMS)
type algorithm as the predetermined criteria which can be expressed
for example in the DFT domain as 4 W = ( k ) E ( k ) X ( k ) xx ( k
) . ( 5 )
[0046] The linearization performed by blocks 58, 72 and 88 is used
to remove cyclic convolution/correlation components produced by the
DFT/IDFT. Blocks 10 and 12 together can also be seen as the
Frequency-Domain Adaptive Filter (FDAF) the derivation of which can
be found in S. Haykin, Adaptive Filter Theory, Prentice Hall,
Chapter 7, 2002, and in E. Ferrara, Frequency-domain adaptive
filtering, in C. Cowan, P. Grant, Adaptive Filters, Prentice Hall,
1985. And finally, block 74 of the gradient adaptation means 12
performs an addition operation to compute the controlled signal 15
required by the echo canceller 10: W(k+1)=W(k)+.DELTA.W(k).
[0047] FIGS. 3 and 4 illustrate one example for realization of the
echo cancellation system 11, according to the present invention.
However, there are many possible variations. For instance, in a
more advanced realization with some modifications, the solution of
the difference Equation (4) could be substituted back into Equation
(1) to find yet a better approximation of the optimum step-size
signal. The whole structure then supports an even more precise and
closed solution to the acoustic echo control problem. The approach
as discussed above was basically thought as an intuitive
realization example of the present invention. This also implies the
steps of operation as shown in FIG. 5.
[0048] FIG. 5 shows a flow chart illustrating a performance of a
statistical adaptive-filter controller of FIG. 3 to optimize echo
cancellation. In a method according to the present invention, in a
first step 100, a microphone signal y(i) 28, a part of which is the
echo signal d(i) 22, is provided by the microphone 18 and the voice
signal x(i) 20 is provided by a telephone receiving path. Said echo
signal is a microphone response to an acoustic output signal
provided by a loudspeaker 16 in response to the voice signal x(i)
20. In a next step 101, constant .vertline.G'.vertline..sup.2 is
pre-selected by the SAFE 40a of the SAFC 40. In a next step 102,
the power spectral density signals .PHI..sub.xx(k) 46 and
.PHI..sub.ee(k) 48 of the voice signal x(i) 20 and echo reduced
microphone signal 34, respectively, are determined. In a next step
104, the step-size signal .mu.(k) is determined using Equation (3)
by the SAFC 40a. In a next step 106, the residual echo power
transfer function .vertline.G(k).vertline..sup.2 is determined by
solving Equation 4 and the postfilter weights signal H(k) 44 is
determined using Equation 2 by the SAFE 40a. In a next step 108,
the step-size signal .mu.(k) 15 is provided to the gradient
adaptation means (GAM) 12 and the postfilter weights signal H(k) 44
is provided to the postfilter 14 by the SAFE 40a. After the step
106, the process continues to steps 110 and 116 which initiate two
procedures taking place in parallel.
[0049] In a step 110, the gradient signal AW(k) of the further
controlled signal 15 is determined using Equation (5) by the
gradient adaptation means 12 which further performs an addition
operation W(k+1)=W(k)+.DELTA.W(k) and further provides W(k+1) to
the echo canceller 10. In a next step 112, the echo canceller 10
provides the estimate echo signal d'(i) to the adder 30. In a next
step 114, the echo reduced microphone signal e(i) 34 is generated
by the adder 30 and provided to the GAM 12, to the postfilter 14
and to the power spectral density means 40b of the SAFC 40.
[0050] In a next step 116, the postfilter 14 further reduces the
residual echo component of the echo reduced microphone signal 34
using the postfilter weights signal H(k) 44 which weights the echo
reduced microphone signal 34 for generating a high quality
undistorted (strongly echo reduced) microphone signal 36.
[0051] After steps 114 and 116, in a next step 120, a determination
is made whether communication (e.g., phone conversation) is still
on. If not, the process stops. If communication is still on, the
process returns to step 102.
* * * * *